Quasistellar Objects: Overview ENCYCLOPEDIA of ASTRONOMY and ASTROPHYSICS

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eaa.iop.org DOI: 10.1888/0333750888/1585

Quasistellar Objects: Overview Patrick Osmer

From Encyclopedia of Astronomy & Astrophysics P. Murdin

ISBN: 0333750888

Institute of Physics Publishing Bristol and Philadelphia

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Quasistellar Objects: Overview

Quasistellar objects, or quasars, were defined originally as star-like objects of large redshift. Quasars are believed to be powered by the accretion of matter onto massive black holes at the centers of galaxies, a process that emits more energy than thermonuclear reactions. Today, quasars are considered to be the most luminous members of the general class of objects called active galactic nuclei, or AGNs. Quasars are the most luminous objects in the universe. This article begins with a brief history of the discovery of quasars. Next it describes their main properties and the concepts that have been developed to explain them. It continues with a description of their nature and theoretical models. Then additional properties and topics are considered: absorption lines, host galaxies, Figure 1. Optical image of 3C273. The object looks stellar in and luminosity functions and evolution. this image except that it is accompanied by a jet of radiation extending to the lower right. (Credit National Optical Astronomy Observatories/National Science Foundation. History Copyright Association of Universities for Research in In 1960 Mathews and Sandage identified the radio source Astronomy Inc (AURA), all rights reserved.) 3C 48 (the 48th object in the 3rd Cambridge catalog of radio sources) with a star-like object of 16th magnitude. by more than a factor of 4. The light from 3C 9 had taken Its optical spectrum showed unusual emission lines that 80% of the age of the universe, or more than 10 billion could not be identified, and the object was considered to years, to reach Earth. The ability to observe objects at be a radio star, more of which were identified in the redshifts greater than 2 meant that astronomers could course of their work. In 1962, Hazard, Mackey and probe back in time to within a few billion years of the big Shimmins established an accurate radio position for bang. another source, the BRIGHT QUASAR 3C 273, by observing The unusual nature of quasars was compounded by the object as it was being occulted by the Moon. Schmidt the work of Smith and Hoffleit, who showed that 3C 273 used this position to identify 3C 273 with a 13th varied significantly in brightness on timescales of magnitude star-like object (figure 1), whose spectrum months. This variability indicated that the characteristic also displayed unusual emission lines. Schmidt size of the emission region was light-months and led subsequently realized that the emission lines could be immediately to the question, how could such a small identified with lines from hydrogen and oxygen object produce more light than an entire galaxy, with a redshifted by 15.8% of their rest wavelength. A redshift typical size of 100 000 light-years? of 0.158 was unprecedented for such a bright object. If To summarize, quasars revolutionized astronomy in the redshift arose from the expansion of the universe, it the 1960s for two reasons: (1) they pushed back the meant that 3C 273 was about two billion light-years limits of the observable universe significantly in both distant, according to the Hubble law, in which the distance and lookback time and (2) their compact size redshift of objects increases in proportion to their and great luminosities could not be explained in terms of distance. Furthermore, the combination of the observed the stars and galaxies known at the time. Quasars opened brightness of 3C 273 with the inferred distance meant new frontiers in cosmology and stimulated the that it had an intrinsic luminosity 100 times that of an development of a new subject—relativistic astrophysics. entire galaxy like the Milky Way. Following Schmidt’s work, it was immediately Main properties and concepts realized that the redshift of 3C 48 was 0.37. By 1965, 3C The main observed properties of quasars are the 9 was found to have a redshift of 2.01, well in excess of following. the 0.46 value for 3C 295, the most distant galaxy known 1. Nuclei that appear starlike in optical images. at the time. It had taken more than 30 yr of work on Extended emission can now often be detected around galaxies with the largest telescopes in the world for the nucleus, and jets extending away from the astronomers to push the limit of the observable universe nucleus are occasionally seen. to the distance of 3C 295. Only 2 yr after the discovery of 2. Spectra showing broad emission lines with widths quasars, the maximum observed redshift had increased greater than observed in normal galaxies and a range

of ionization broader than seen in typical nebulae (see BL LACERTAE, BL LACERTAE OBJECTS and AGN: ionized by stars. VARIABILITY). 3. Structures in quasars with observable radio emission Variability provides powerful tools for determining that range in angular extent from m arcsec to tens of the inner structure and nature of quasars. When the arcsec. The most compact structures are often central source itself varies in brightness, the effects of the observed to expand on timescales of years. variation propagate outward with time and produce 4. Radiation of similar power at energies ranging from changes in, for example, the emission line spectra and γ-rays to the far-infrared (100 µm) region of the continuum radiation from the material surrounding the spectrum, with a decrease in energy at radio central source. In the case of blazars, we are seeing frequencies. almost directly down the axis of the jets of material that 5. Redshifts, z, of spectral features ranging from 0.1 to are being ejected from the central regions. By following 5. how the radiation from the jets varies at different 6. Variability in brightness at different wavelengths on wavelengths with time, we are able to infer the physical timescales as short as days to weeks. conditions and processes that are occurring in the jets. 7. Luminosities as high as 1014 Suns. Observations of the variability of quasars carried out 8. Total energy in high-energy electrons in the region at different wavelengths in intensive campaigns and also of radio emission that can exceed 1060 erg. in programs extending for years have provided some of Additional information about these properties and the the most direct information on the nature of the inner concepts they imply about the nature of quasars follows regions in quasars and AGNs. These programs have below. shown that the emission line regions are considerably smaller and closer to the central source than was Redshifts originally thought. The large redshifts of quasars are one of their distinctive features, and the value of the greatest known redshift, z, Continuum emission and multi-wavelength continued to increase after 1965, surpassing z = 3 in 1973 observations and z = 4 in 1987. The maximum value reached z = 4.9 in Quasars emit radiation strongly in bands ranging from γ- 1991, and an object at z = 5 was discovered in the first rays and x-rays to the far infrared (100 µm). The amount stages of the Sloan Digital Sky Survey in 1998 (see of energy emitted in each band by most objects is QUASISTELLAR OBJECTS: SURVEYS). The great majority of remarkably similar, in contrast to normal thermal quasars, however, have redshifts less than 2.5. This radiation from stars, which is much more peaked and occurs for two reasons: (1) such quasars are easier to find restricted in wavelength. For example, most stars are cool because they are brighter in the ultraviolet than most stars and are faint at ultraviolet wavelengths. In contrast, most and (2) they are intrinsically more numerous than quasars quasars with redshifts less than 2.5 are bright at with z > 2.5. The low-redshift limit of quasars is ultraviolet wavelengths, a property that is very helpful in normally set at 0.1 because objects at lower redshift are distinguishing them from the more numerous stars that usually resolvable as galaxies and are cataloged as are found in sky surveys. ACTIVE GALACTIC NUCLEI. Although quasars were discovered through their In addition to providing our first view of the radio emission, about 90% of quasars do not emit universe at z > 0.5, quasars also proved to be valuable strongly at radio wavelengths and are classified as radio cosmological probes because they were luminous enough quiet. Radio-loud quasars are typically 100 times brighter to yield clues on the properties of matter along the line of at radio wavelengths than radio-quiet quasars. sight back to the Earth (see discussion on absorption lines The continuum emission in quasars appears to arise below and in more detail in QUASISTELLAR OBJECTS: from a combination of thermal and non-thermal INTERVENING ABSORPTION LINES). processes. For the latter, synchrotron emission from relativistic electrons produces the lower-energy radiation. Variability The x-ray and γ-ray emission may come from the inverse Observations indicate that variability is a common Compton scattering of lower-energy photons by high- feature of quasars and most quasars are believed to show energy electrons, although thermal mechanisms may also variations in light at the 10–40% level over timescales of play a part in x-ray emission. In any event, the continuum months to years. Furthermore, variations are observed in radiation from quasars demonstrates that some very the strength and shape of emission lines and in energetic processes are involved. Furthermore, the continuum emission at x-ray, ultraviolet and radio continuum radiation at the highest energies tends to show wavelengths. The variations are irregular in nature, with the most dramatic variability and the shortest timescales, periods of flaring and relative quiescence being which is another indication of the extreme conditions that intermixed. Some objects, which are called BLAZARS, exist near quasars. show variations in light on timescales as short as 1 day Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 2 Quasistellar Objects: Overview ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS

Emission lines The strongest emission lines in quasar spectra come The emission line spectra of quasars (figure 2) are from hydrogen, carbon and magnesium, with lines of characterized by the large widths of the lines and wide nitrogen, oxygen, iron and other elements also being range of ionization. The full widths at half-maximum of visible. The observed levels of ionization range from the emission lines are typically 5000 km s−1 and range neutral for hydrogen and oxygen up to five-times ionized from about 500 km s−1 to greater than 10 000 km s−1. Full oxygen and even more highly ionized iron. The range of widths at the base of the lines can exceed a tenth the ionization is another indicator of the strength of the non- speed of light. thermal continuum radiation in quasars and its extent to The widths are produced by motions of gas in the high energies. emitting region, where a mixture of infall, rotation and Although it is difficult to make a reliable ejection probably occurs. The widths are consistent with determination of the chemical abundances in quasar the emission region being at a distance of light-months to emission line regions, results to date indicate that the a few light-years from a central black hole. abundances are similar to those observed in present-day One of the most striking observational properties of stars and nebulae, even in quasars at the highest redshifts. the spectra of quasars and AGNs is the anticorrelation of This is a striking result when one considers that the most the strength of the C IV emission line at 1550 Å and the distant quasars correspond to cosmic times of 1 or 2 continuum luminosity, that is the emission line is billion years after the big bang. Except for hydrogen, all observed to be weaker relative to the continuum in more the elements mentioned above originate in the luminous objects. This relation, which is called the thermonuclear reactions that occur in stars. The Baldwin effect, is perhaps the most prominent observed abundances of these elements increase with cosmic time correlation that has been found for quasar and AGN as successive generations of stars complete their spectra. It is important for two main reasons: (1) the clues evolution. When stars die, they return part of their newly it provides about the nature of the emission line region produced elements to interstellar space, and the next and central source and (2) the potential it offers for using generation of stars forms from the enriched material. quasars as cosmological probes at high redshifts. Thus, the apparently normal abundances of the elements in high-redshift quasars implies that they were preceded by substantial stellar activity in the early universe.

Nature The prodigious luminosities of quasars in combination with their small size led theorists to consider gravity as their energy source immediately on their discovery. It was realized that normal stars and thermonuclear reactions were entirely insufficient to produce the observed properties of quasars. However, the required gravitational potential energy could only be extracted if quasars contained compact objects with masses of a hundred million Suns and sizes about that of the solar system. In addition the gravitational field of such objects was so strong that the effects of general relativity would be dominant in their vicinity. Since that time, gravity has remained as the consensus source of energy for quasars, although the concepts of the nature of the central source have changed considerably. For example, supermassive ‘stars’, were originally postulated as the central objects in quasars, but enough difficulties were found with such ideas that they were soon abandoned. Subsequently, it was realized that BLACK HOLES were the most likely central engines of quasars. A black hole was an object with gravity so strong that not even light Figure 2. A composite quasar spectrum covering the ultraviolet could escape from its surface. The widths of the emission region of the spectrum. It shows strong emission lines from lines in quasars and estimates of the size of the emission hydrogen, carbon, nitrogen and magnesium. (Credit Francis P et line region suggested that the central mass was about 108 al 1991 Astrophys. J. 373 465.) Suns. Such objects would not be able to support themselves against the pull of their of their own gravity, Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 3 Quasistellar Objects: Overview ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS and they would collapse to form a black hole. The of the motion toward the Earth. See the article on collapse could liberate 10% or more of the rest energy, SUPERLUMINAL MOTION for more information. mc2, of the object, that is, more than 10 times the energy The emission line spectra of quasars indicate that of the thermonuclear reactions that occur in stars. there are also gas clouds surrounding the black hole. The Similarly, any material later captured by the black hole clouds occupy about 10% of the volume of the region would liberate the same fraction of rest energy. This of where the broad emission lines are produced. The broad course provided an explanation for the energies found in emission line region in quasars can extend to several quasars. As a result, quasars stimulated much work on light-years from the black hole. the origin and nature of black holes and the detailed Understanding the nature of the inner regions of processes of how the observed energy could be obtained quasars and the central black holes continues to be an from them. important subject of research both observationally and Today, the working hypothesis for quasars and theoretically. Specific topics include the formation of AGNs is that massive (106–109 solar masses) black holes black holes and their relation to the formation of exist at their centers. The radiated energy we detect from galaxies, the role that galaxy interactions play in them comes from matter being accreted onto the black providing the fuel supply of gas for accretion onto the hole. According to general relativity, the radius of a black black hole and how quasars evolve in time and what is hole is 3 km per solar mass, and the radii of black holes their relation to the inactive black holes now being in AGNs and quasars range from a few solar radii to 20 observed in the centers of nearby galaxies. times the distance of the Earth from the Sun. It is most likely that much of the matter surrounding Alternative theories the black hole is in the shape of a disk. The matter in the One of the first alternative explanations for the large disk orbits the black hole and moves inward as it loses redshifts of quasars to be investigated was gravitational. angular momentum from some source of viscosity in the If the emission arose near a compact massive object with disk. The inner edge of the disk may extend to within a a sufficiently strong gravitational field, then the emission few radii of the central black hole. The gas in the inner lines could exhibit large redshifts. However, it was regions of the disk is expected to be hot enough, about realized almost immediately that such a hypothesis was 4–6 10 K, to account for the thermal component of the inconsistent with the widths and other properties of the continuum radiation at ultraviolet wavelengths. emission lines, and the hypothesis was ruled out. The x-ray observations of quasars provide evidence Another hypothesis was that quasars were being of the existence of a corona of energetic particles above gravitationally lensed and thus appeared much brighter the accretion disk. The x-rays may be produced by than they really were. Interestingly, many years after this upscattering of the ultraviolet photons from the disk on idea was proposed, some quasars were discovered to be the energetic particles via the inverse Compton process. lensed, but they are only a small fraction of the total. Some x-rays may also be produced in shocks located From observations of the apparent associations of farther from the central source. quasars with galaxies of much lower redshift, Arp has Jets of beamed radiation are also an important challenged the concept that quasar redshift arise from the feature. They are frequently observed in radio-loud cosmological expansion of the universe. In this picture, quasars and, as in the case of 3C 273, sometimes are seen the redshifts arise from some other, non-cosmological at optical wavelengths. The jets are thought to be effect whose nature has yet to be determined. accelerated electromagnetically and ejected along the Subsequent observations, however, have shown that rotational axis of the black hole. They can be moving at quasars do reside in galaxies. In the cases where it has relativistic speeds and contain high-energy particles, such been possible to measure the redshifts of the host as the relativistic electrons that produce the radio galaxies, they agree with the redshifts of the quasars. emission through the synchrotron process. The radio Furthermore, low-luminosity quasars and luminous emission in the 3C 273 jet is located several light-years Seyfert galaxies overlap in luminosity and show a distant from the central source. general continuity of properties. A striking feature of jets in some radio sources is Today, it is generally accepted that the large that repeated observations on the smallest angular scales redshifts of quasars do arise from the expansion of the with radio interferometers show them to be moving universe and that they are at the distances indicated by outwards at speeds that appear to be several times greater the redshifts. than the speed of light. This of course would violate a A different approach has been taken by Terlevich basic limit of relativity. The most satisfactory and collaborators, who have proposed that starbursts, explanation for this paradox is that the jets are moving at intense episodes of star formation, can account for speeds near that of light in the direction of the Earth. In quasars. In this case energy from hot, young stars and the this case the apparent time interval in the expansion supernova explosions that occur at the end of their motion of the jet appears smaller than it really is because evolution provides the radiation and spectral properties

Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 4 Quasistellar Objects: Overview ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS observed in quasars and AGNs. While there are abundant of the gas. Studies of absorption lines in quasars have observations showing that starbursts are common in provided crucial information about gas in the high- regions around AGNs and may well be related to the redshift universe. Intervening systems may themselves be processes that fuel the central object, the starburst model classified in three groups according to the column density has great difficulty in accounting for the most luminous of the absorbing gas: (a) damped Lyman α systems, (b) quasars and for the rapid variability in x-rays that is intermediate (metal line and/or Lyman limit) systems and observed in AGNs. Furthermore, the observational (c) the Lyman α forest. evidence for black holes in nearby galaxies has become Damped Lyman α systems represent the highest increasingly strong. column densities, typically in excess of 1020 neutral hydrogen atoms cm−2, and are so called because Additional topics absorption is so strong that the damping wings of the line are seen. Such systems are thought to occur when the line Absorption lines of sight passes through a disk galaxy. Some may also Absorption lines were detected in quasars within a few arise in dense clouds or filaments of intergalactic gas at years of their discovery and were initially believed to be high redshift. rare. However, with improved sensitivity and spectral Damped Lyman α systems, because of their high resolution, it became clear that absorption lines were column densities, also exhibit absorption lines from present in all quasars with redshifts 2 and greater. A heavier elements such as carbon, silicon, magnesium and contributing factor to their presence was that ultraviolet, iron, with additional elements being detected in the most ground-state lines were being redshifted to wavelengths favorable cases. Analyses of these lines indicate that the where they could be observed with ground-based abundance of metals in the systems is about one-tenth of telescopes. Today, the study of quasar absorption lines is the solar value. This is consistent with our understanding a major topic and the article on this subject should be of the chemical evolution of galaxies, in which the consulted for more details. abundance of metals increases with cosmic time owing to Absorption lines may be placed in three categories the continuing enrichment of the interstellar medium by according to the distance of the absorbing gas from the supernovae and red giant stars as they complete their life central emission source of the quasar: (1) intrinsic cycle. Damped Lyman α systems can only be observed systems, (2) associated systems and (3) intervening from the ground for redshifts greater than 1.8, which systems. corresponds to a lookback time of approximately 80% of Intrinsic systems arise close to the quasar itself. The the age the universe. most conspicuous members of this class are the broad Intermediate systems, with a characteristic column 17 −2 absorption line (BAL) quasars, whose spectra show density of 10 neutral hydrogen atoms cm , also show broad troughs of absorption on the short-wavelength side absorption lines from elements such as carbon, of the Lyman α line of hydrogen and such high- magnesium and other metals plus a discontinuity at the ionization lines as C IV λ1549 and N V λ1240, for Lyman limit of hydrogen. They are commonly attributed example. The troughs indicate outflow velocities that can to the halos of galaxies. Their metal abundances range exceed 30 000 km s−1. BAL features are seen in about down to 1% of the solar value, indicating that they have 10% of high-redshift quasars, a value that gives an had less chemical enrichment than the gas in damped indication of the average covering factor of the Lyman α systems. outflowing gas. The Lyman α forest lines are the most numerous and Associated systems show absorption features from ubiquitous absorption features and are attributed to elements such as hydrogen, carbon and magnesium. They clouds of gas in the intergalactic medium. They are seen have redshifts close to that of the emission lines in in a range of column densities beginning at the limit of 12–13 −2 quasars but are much narrower. They are believed to detectability around 10 neutral hydrogen atoms cm arise in gas associated with the host galaxy of the quasar and extending up into the range of the intermediate or the environment in which the host galaxy resides, systems. Their number density increases with redshift; by which might be a group or cluster of galaxies. redshift 4, the integrated effect of absorption from the Intervening systems have redshifts smaller than the Lyman α forest produces a significant depression of the emission lines in quasars and arise in clouds of gas flux shortward of the Lyman α emission in quasar unrelated to the quasar that lie along the line of sight to spectra. Observations of the Lyman α forest are used to the Earth. In this case, the quasar serves as a background deduce the state of ionization of the intergalactic beacon that enables the study of otherwise invisible gas. medium. Much of the ionizing flux at high redshift is The sight lines to quasars probe representative samples of produced by quasars. The data are also important for intergalactic space, and absorption line spectra provide understanding the formation of structure in the universe. an unbiased way to determine the nature and distribution

Figure 3. A sample of images showing the host galaxies of quasars. The hosts can be normal-looking spiral and elliptical galaxies (left panels) but are frequently seen to be interacting or merging galaxies (center and right panels). (Credit J Bahcall, M Disney, NASA. This image was created with support of Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc from NASA contract NAS5-26555, and is reproduced with permission from AURA/STScI.)

and evolve. The quasar is in the nucleus of the galaxy Host galaxies and represents the endpoint of the process that produced The regions immediately around quasars are difficult to the central concentration of matter in the galaxy. How a observe because of the glare from the central source, massive black hole formed and grew from the central which is why quasars were originally called star-like concentration remains an important research question. At objects. However, with improved observations, extended the same time, for the black hole to be visible and attain emission began to be detected around quasars. In the the luminosity observed for bright quasars, it must meantime the realization was developing that quasars, accrete several solar masses of material per year. What Seyfert galaxies and other active galactic nuclei were causes this inflow of matter from the outer parts of the members of the same family. One of the main differences galaxy to the nucleus is also an important topic. It is among the different classes of objects was the luminosity thought that the interaction of two galaxies, that is their of the central source, and the host galaxies with low- collision or near collision, can produce enough disruption luminosity nuclei were much easier to study. to the orbit and angular momentum of stars and gas to The excellent image quality and spatial resolution of produce the inflow, but the details of the process are not the Hubble Space Telescope have enabled a significant yet fully understood. advance of our knowledge of quasar host galaxies (figure 3). Results to date for low-redshift objects show that Luminosity functions and evolution quasars occur in elliptical, spiral and interacting galaxies, The luminosity function of quasars is the distribution of with some of the latter appearing very disturbed by the their volume density as a function of luminosity and interaction. On average, the host galaxies are brighter redshift. As with galaxies, the number density decreases than normal field galaxies. The radio-loud quasars tend to steeply at the highest luminosities. At low luminosities occur in elliptical or interacting galaxies. The radio-quiet the slope of the function is smaller than at high quasars can occur in elliptical or spiral galaxies. These luminosities, so the rate of increase of objects slows with results are in contrast to the situation for lower- decreasing luminosity. luminosity AGNs, where there are stronger tendencies Luminosity functions are among the most basic for radio-quiet objects to occur in disk galaxies and parameters in astronomy and provide a basis for radio-loud objects in ellipticals. determining the amounts of matter in different forms in The study of host galaxies is important to improving the universe. our understanding of how both galaxies and quasars form

One of the most striking properties of quasars is the evolution of their luminosity function with redshift. This was first noticed by Schmidt, who realized there were too many quasars at high redshift in early surveys if the space density of objects were constant. Interpreted as density evolution, the space density of luminous quasars is more than a thousand times greater at z = 2 than in the local universe. An alternative explanation is that quasars were brighter at high redshift than at present. At redshifts greater than 3, there is considerable observational evidence that the space density of quasars declines steeply. A logical interpretation of the results (figure 4) is that we are seeing back to the epoch of peak quasar activity. Alternatively, clouds of dust at high redshift could be blocking our view of more distant quasars. For example, such clouds could be associated with star formation regions in the host galaxy of the quasar. However, the best information currently available indicates that dust is an important factor for some types of quasars but does not account by itself for the decline in space density at high redshift. In any event, the most distant objects now known in the universe are galaxies, the first time this has happened since 1965. Although this may be a coincidental result, it is another indication that quasars at redshifts greater than 5 are genuinely very rare objects. Current observations of quasars and distant galaxies Figure 4. The evolution of the space density of luminous are consistent with bursts of star formation occurring in quasars as a function of cosmic lookback time, expressed as a galaxies before the quasar activity builds up to its peak fraction of the age of the universe. The space density shows a around redshift 2–3. As was mentioned earlier, the gas in strong peak at 0.85, when the universe was 15% of its the emission line regions of distant quasars appears to present age. (Credit Warren S, Hewett P and Osmer P 1994 have relatively normal abundances of elements heavier Astrophys. J. 421 412.) than hydrogen, which implies that the gas has been enriched by massive stars before reaching the quasar. However, the picture is but a sketch and is in need of Furthermore, it takes time for a supermassive black hole both development and confirmation. We may expect to attain the masses needed for quasars, time that allows significant progress in the near future on several fronts. for a few generations of massive stars to complete their • Large new surveys for quasars that are being carried evolution. For reference, the age of the universe is only out at optical and radio wavelengths, such as the about a billion years at redshift 5. Sloan Digital Sky Survey, the Australian 2dF Survey Subsequently, the overall rate of star formation in the and the FIRST radio survey, will increase the universe reaches a peak as galaxies continue to form and number of known quasars by a factor of ten. (In June assemble. After that, both quasar activity and star 2001, two quasars with redshifts of 6.0 and 6.2, the formation rates decline until they reach the values most distant objects ever observed in the universe observed today in the local universe. Both declines are were identified in the Sloan Digital Sky Survey.) probably related to galaxies completing their formation • The new generation of large telescopes with 8–10 m and assembly. In addition, the rate of interactions and apertures that has come into operation and space mergers of galaxies decreases as the universe continues observatories such as Chandra and XMM that will to expand. As a result less material is available (1) to fall soon be launched provide powerful tools for to the central regions of galaxies and fuel the massive observing quasars with significantly more sensitivity black holes and (2) to form stars. and resolution at optical, infrared and x-ray In this picture, the quiescent black holes that today wavelengths than has been available previously. are found to be ubiquitous in nearby galaxies are the • Advances in theoretical modeling and the continuing products of the spectacular galaxy formation process and increase in computing power will enable significant quasar activity that occurred in the first third of the progress in interpreting the new observations and in history of the universe. modeling the inner regions of quasars and key processes such as accretion.

Thus, the prospects for improving our understanding of how quasars form and evolve and for determining their relation to galaxies are very bright.

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